Add experimental infix macro support

Experiment is experimental.

Macros now take a second argument which is a single element list with the previous term.

In the old pre-1.0 days we allowed a list of all the previously enforested terms but this didn't behave well so I'm starting out with just a single term. We can argue our way into relaxing this restriction later.

Note that infix macros are contained within implicit delimiters so:

syntax m = (ctx, prev) => {
  return prev;
}
1 + m

throws an error since prev is the empty list.